McMillan Airfield GPS origin is not correct

[Jaeyoung-Lim]

When comparing to the map, the origin of the GPS coordinates are not correct and seems to be off a few hundred meters.

I tried fixing this comparing on google maps, but changing
https://github.com/PX4/sitl_gazebo/blob/12da9e6104aaf72a0c52bf802644310f0c5b1ee1/worlds/mcmillan_airfield.world#L18-L23

Didn't have an effect on setting the gps origins. It seems like https://github.com/PX4/sitl_gazebo/blob/master/src/gazebo_gps_plugin.cpp#L51 is not able to be modified.

@ahcorde Is there somewhere else that the world origins are set? For other worlds changing the spherical coordinates works as expected. but the mcmillan airfield seems to not take the spherical coordiates defined in the mcmillan_airfield.world file

[Jaeyoung-Lim]

Posted issue in gazebosim/gazebo-classic#2884